Low-temperature solution-processed organic-inorganic halide perovskites have demonstrated striking performance in photon harvesting photovoltaics with efficiencies exceeding 22.1%. Additionally, these hybrid materials have displayed great potential for light-emitting applications as a result of their large optical absorption, high photoluminescence quantum yield (PLQY). Especially, the colloidal nanocrystals (NCs) of organic-inorganic and all-inorganic halide perovskites have exhibited further improved light-emitting performance due to the quantum confinement effect, which results in increased PLQY, tunable optical bandgap and PL wavelength.
In comparison to the conventional II-VI group semiconductor nanocrystals (like CdSe), the halide perovskite nanocrystals exhibits one to two orders larger one-photon absorption cross-sections, resulting from the stronger light-matter interactions in the perovskite NCs. Such strong light-matter interactions imply large nonlinear optical properties (especially multiphoton absorption) of the perovskite NCs, which is essential for applications in optical-limiting, multiphoton microscopy for deep tissue imaging and low-threshold upconversion lasing.
Materials or devices possessing large transmission at low incident light intensity/fluence, while exhibiting small transmission at high incident light intensity/fluence are referred as optical limitters. In the optical limiters, the light transmission decreases nonlinearly as the incident intensity/fluence increases, and drops significantly when the intensity/fluence of the incident radiation exceeds a certain threshold, the optical limiting threshold. As per convention, the intensity/fluence at which the light transmission decreases to 50% of the linear transmittance is defined as the limiting threshold. In accordance with such intriguing properties observed in optical limitters, they are valuable for protecting optical sensors, human eyes and other light-detecting objects, which may suffer from damages resulting from direct exposure to strong light sources such as pulsed lasers. Although NCs of certain metals and metal oxides possess optical limiting properties, they are not suitable for use in short laser pulses with nanosecond or femtosecond pulse widths. Optical limiting materials based on two-photon absorption (TPA) with large TPA properties and thus low-limiting threshold have been demonstrated to be appropriate for optically limiting nanosecond or femtosecond pulsed lasers.
In contrast to linear emission results from one-photon absorption, upconversion PL excited by simultaneous multiphoton absorption possesses essential merits for deep tissue imaging, including higher spatial resolution, larger penetration depth, less light scattering and less damage to the investigated samples. Furthermore, in contrast with the upconversion PL excited by lower-order nonlinear optical absorption, the four- and five-photon absorption processes with four- and five-order dependence on the incident light intensity/fluence can provide much stronger spatial confinement, resulting in much higher contrast in imaging.
However, due to the low transition probabilities of the four- and five-photon absorption processes in both organic molecules and conventional inorganic semiconductors, it is notoriously difficult to observe their related phenomenon.
Multiphoton pumped upconversion lasing not only possesses all the above merits of upconversion PL, but is also valuable for applications in effective frequency upconversion and short-pulse optical communications, owing to its coherent nature and the merit of no requirement for phase-matching conditions. Since large enough multiphoton absorption is required to achieve population inversion in the gain medium, realizing multiphoton pumped lasing is more challenging and difficult than the upconversion PL.
In this context, core and core-shell type organic-inorganic perovskite NCs with different sizes and shapes are targeted as competitive optical limiting materials working in the short laser pulses region (nanosecond and femtosecond), due to their excellent two-photon absorption properties. Importantly, owing to their large multiphoton absorption and high PLQY, these samples are also applied for achieving efficient multiphoton excited upconversion PL and low-threshold upconversion lasing.
Organic-inorganic hybrid halide perovskite materials have already revolutionized solar cell applications. The certified power conversion efficiency of solar cells based on CH3NH3PbI3 and derivatives have reached a record value of 20.8% within a short period. Concurrently, the potential of perovskites to transform the field of light emission has been demonstrated. The focus has now been expanded to encompass the fabrication of perovskite nanocrystals (NCs) to achieve color tunability and enhanced PLQYs. It appears that even a slight change in the synthetic protocol can significantly influence the band-gap tuning, size, ordering and photophysical properties of the nanocrystals. For example, the Perez-Prieto group reported a non-template strategy for the synthesis of colloidal methylammonium lead bromide (MAPbBr3) nanocrystals with a particle size of around 6 nm showing a photoluminescence quantum yield (PLQY) of up to 83%, whereas the Dong group reported a ligand-assisted re-precipitation strategy (LARP) for the synthesis of MAPbBr3 nanocrystals (NCs) with a particle size of around 3.3 nm and photoluminescence quantum yield (PLQY) between 50 and 93%. Despite these promising results, efforts have not been made to improve luminescence stability in parallel with enhancing the PL efficiency.
It is therefore an object of the present disclosure to provide a material with nonlinear optical properties showing upconversion photoluminescence excited by simultaneous multiphoton absorption. It is also an object of the present disclosure to provide a material with a high quantum yield which remains stable over a long time.